There has been a trend toward the use of control and regulating systems in motor vehicles including engine and transmission controllers, anti-lock systems (ALS), anti-slip control systems (ALC), and multi-functional vehicle dynamics control systems. Powerful bus systems may be needed to exchange data in real time between these components. A bus system referred to as the Controller Area Network (CAN) was developed for optimum handling of these functions. CAN is also used in industrial applications beyond the field of automotive engineering.
The CAN bus is a serial bus to which the individual stations, for example, control units, are connected via CAN controllers. The individual stations are able to communicate with other stations over the bus by sending and receiving messages.
Much of the data transmitted in motor vehicles may be related to safety. The data may be used to control the on-board safety systems, for example, the brake system. In the case of safety systems, it is important to prevent the occurrence of situations that could jeopardize safety in the event of a malfunction. It must therefore be ensured that safety-related data does not become corrupted during transmission, or that corrupted data is detected.
Each time a message is transmitted, it is possible—albeit not very probable—for one bit within the message to change value, for example a “1” switching to a “0”. It is even possible for multiple bits within the same message to “flip over.”
The main causes of such disturbances may be external electromagnetic fields that may be unavoidable in technological systems.
Because it may not be possible to prevent errors, i.e., a bit “flipping over,” it may be desirable to reliably detect the error. This may be done by the sender inserting redundant check bits into the message. The receiver can then use the check bits to determine whether the transmission was error-free or faulty.
The CAN controllers may support very reliable data transmissions. The data is provided with CRC check bits during transmission. These bits are verified on receipt. This may make it possible to detect faulty transmissions and respond to them accordingly. For example, data that was not properly received the first time around may be requested again.
The actual interface between a CAN controller and a CAN bus is the CAN protocol kernel. It is used to transmit data to the bus and receive data from the bus. During transmission, the CAN protocol kernel generates the check bits and sends them together with the corresponding data. On receipt of a message, the CAN protocol kernel checks the received check bits and thus determines whether the data was transmitted without errors.
The CAN-based CRC check bits monitor only especially error-prone transmissions between the controllers. However, transmission errors cannot be ruled out even during transmissions within the CAN controller and during transmissions between the CAN controller and a microprocessor, for example, the central processing unit (CPU). This may be a weakness in the ability of conventional CAN controllers to monitor data transmission errors.
To secure internal data transmissions, conventional systems may use, for example, a “loop-back mode.” The CAN bus output and the input of the CAN protocol kernel may be interconnected, while the CPU may transmit defined data and receive it again directly. A comparison of the transmitted and received data may enable the entire internal transmission link to be checked.
The disadvantage of this method may be that the control unit may need to be disconnected from the bus during checking to avoid making the check data visible on the bus. In many systems, temporary disconnection of a station is unacceptable, which means that a loop-back test may be carried out only once at activation.
The method described above thus may not support continuous monitoring of the entire transmission path. However, this may be necessary for safety-critical systems.